Argentite

Argentite was primarily treated as a mineral belonging to the galena group, cubic silver sulfide (Ag₂S), occasionally found as uneven cubes and octahedra, but more often as dendritic or earthy masses, with a blackish lead-grey color and metallic lustre. All these forms are structurally not argentite, but its monoclinic polymorph, acanthite. Due to instability in normal air temperature, the IMA commision decided to reject this mineral, which now is a discredited mineral species. The cubic cleavage, which is so prominent a feature in galena, here present only in traces. The mineral is perfectly sectile and has a shining streak; hardness 2.5, specific gravity 7.3. It occurs in mineral veins, and when found in large masses, as in is Mexico and in the Comstock Lode in Nevada, it forms an important ore of silver. The mineral was mentioned 1529 by G. Agricola, but the name argentite (from the Lat. "argentum", silver) was not used till 1845 and is due to W. Haidinger. Old names for the species are Glaserz, silver-glance and vitreous silver. A related Cu-rich mineral occurring i.e. in Jalpa, Mexico, is known as jalpaite.

Acanthite, also Ag₂S, crystallizes in the monoclinic system and is the stable form below 177^oC. As argentite cools below that temperature its cubic form is distorted to the monclinic form of acanthite. Thus, only acanthite is stable in normal air temperature and only this form is beeing found.

References

* [http://www.mindat.org/min-10.html Mindat.org]
* [http://www.webmineral.com/data/Argentite.shtml Webmineral.org]
*1911